This invention relates generally to ultraviolet (UV) cured coatings, and more particularly to UV clearcoat coatings which have demonstrated high flexibility as well as other properties excellently suited for overlay compositions, to processes of their use, and to coated articles resulting therefrom.
By way of background, conventionally cured clearcoat overlays have several disadvantages. In general, the clearcoat and the curing agent are applied separately, in a particular sequence and at particular concentration levels. Because both the proportions of the ingredients and the timing of their application is critical, conventionally cured clearcoat overlays have been difficult and costly to apply, especially with the efficiency and consistency required in modern commercial applications.
Ultraviolet (UV) cured clearcoat coatings overcome several disadvantages associated with conventional clearcoat finishes. In particular, UV cured clearcoat coatings may be premixed so that the polymerization initiator is added to the active ingredients by the manufacturer when the coating is produced, and not by the coating user when the coating is to be applied. Thus, mixing and measurement errors can be avoided and a more consistent product can be obtained.
Despite their many advantages, UV curable clearcoat coating compositions pose problems which must be addressed. For example, typical UV clearcoats are high molecular weight, highly crosslinked films formed from highly reactive acrylate functionality. As such, known UV clearcoats have suffered from limited durability, have been prone to cured resin shrinkage, and have required high doses of UV light to cure. UV clearcoats formulated to overcome these problems typically suffer from a loss of durability or processibility, including diminished scratch resistance, abrasion resistance, weather resistance, chemical resistance, stain resistance, thermal stability and adhesion.
Also, importantly, known UV clearcoats upon curing have long proven to form rigid, unflexible and unyieldable coatings. Thus, problems have been experienced in the industry where clearcoats overlay materials which are prone to expansion or contraction during processing or in the field of use. For example, UV clearcoats overlaying dark, e.g. black finishes which expand upon heating have been known to crack, form other imperfections, or even cause bending or warping of the substrate due to tensions built up in the materials.
In light of the foregoing, a need exists for a UV curable clearcoat composition which is conveniently processible and which forms coatings exhibiting improved physical and chemical properties, e.g., improved flexibility, durability, thermal stability, crack resistance, chemical resistance, stain resistance, weather resistance and adhesion.